After Frenchman Simon, 34, ground down Russian fourth seed Daniil Medvedev 6-7 (4), 6-4, 6-3 in a baseline war of attrition, 37-year-old Lopez used his vast experience to down 18-year-old Canadian Felix Auger-Aliassime 6-7 (3), 6-3, 6-4.
In the men's doubles home favourite Andy Murray, partnered with Lopez in his return from hip resurfacing surgery, continued his dream comeback to make the final by completing back-to-back wins in front of rapturous home support.
As a result left-handed Spaniard Lopez will be contesting two finals on Sunday, including his first in singles since winning the prestigious pre-Wimbledon event two years ago. Simon meanwhile will be aiming to become the first Frenchman to triumph at Queen's.
Both singles semi-finals were absorbing in their own way, while the doubles that followed was filled with tension and excitement.
The huge age difference between Lopez and Auger-Aliassime -- the greatest in any men's singles match since 1977 -- suggested the younger, fresher player would have an advantage if their match went to three sets, but the opposite proved to be the case as Lopez dug deep to come back from a set down and book his place in the final.
Both players squandered two break points but served well in the opener before Auger-Aliassime stepped up in the tie-break, crashing a backhand past Lopez for the mini-break before closing out the set as Lopez slipped at the net.
The match turned at the start of the second as the young Canadian missed a chance to secure a crucial break then dropped serve for the first time all week, before Lopez remained resolute on serve to even up the contest.
In the decider it was Auger-Aliassime who looked jaded and he was broken to love as Lopez went 3-2 up.
The Canadian had a glimmer of a chance to break straight back but missed out and had to save four match points to hold his final service game.
But Lopez was imperious on his own serve and held to love for the match, finishing with a delightful forehand drop shot to win in two hours and 17 minutes.
"I'm so happy, I can't believe I'm in the final here again. It's crazy how we old guys can still perform well," Lopez said.
"I feel very lucky to still be competitive at this stage of my career. I would never have imagined I could reach the final of Queen's at 37 years old."
The earlier match between Simon and Medvedev on centre court was crammed full of interminable rallies regularly surpassing 30 strokes as they turned their match into the equivalent of a staring contest.
After requiring three hours and 20 minutes to get past compatriot Nicolas Mahut on Friday -- the longest match at Queen's since 1991 -- it was surprising to see Simon outlast a player 11 years his junior.
"I put the ball in the court. That's what I do. And I do it for long," Simon told reporters later.
With similar styles, both players camped out on the baseline waiting for openings that rarely appeared.
Medvedev moved 4-1 ahead in the opening-set tiebreak before Simon won the next two points, but the Russian then won a 45-stroke rally to move 5-3 ahead on his way to taking the opener.
Medvedev looked ill at ease, however, and after service breaks were exchanged early in the second set it was Simon who began to take the upper hand to level the match.
Simon saved a break point at 1-1 in the deciding set, this time catching the baseline with a backhand to end a hypnotic 49-stroke exchange that had both players puffing out their cheeks on the warmest day of the rain-hit tournament.
Medvedev, clutching his back at times and leaning on his racket like a crutch, looked a spent force and a double fault cost him his serve at 3-4.
Simon sealed victory in emphatic style to become the first French finalist here since Jo-Wilfried Tsonga in 2011.
They had led their British opponents overnight and wasted no time in dispatching them 6-4, 7-6 (3).
Murray and Lopez, whose dream run started when they knocked out top seeds Robert Farah and Juan Sebastian Cabal earlier in the week, stayed on court to immediately begin their semifinal against third seeds John Peers and Henri Kontinen.
In a close first set Murray and Lopez struck the crucial blow by breaking at 5-5, before Murray served out with ease, finishing off with a trademark ace.
The pairs exchanged breaks in the second before a perfect cross-court forehand from Murray gave him and Lopez a chance to serve for the match at 6-5.
But Murray then looked nervous on serve and was broken as Lopez was peppered by Peers and Kontinen at the net.
And Peers and Kontinen looked rejuvenated in the tie-break, taking the game to Murray and Lopez and finally levelling up on their fourth set point.
That took the semifinal to a championship tie-break in fading light in West London, and as both pairs allowed errors to creep in it was Murray and Lopez who showed the greater consistency to finally take the match 7-5, 6-7 (6), 10-8.
"It was a good match. We obviously had some chances to finish it in the second set," said Murray.
"It was an unbelievable effort from Feliciano. He's played a lot of tennis in the last couple of games. He's not young anymore.
"I'm very happy to be in the final."