I think trying some assorted 'wiggling and jiggling' around of the parts during reassembly is what's going to do it for you.
Of course you insert one pin at a time. Just to clarify a point, I feel sure that the pins aren't tapered so they can be installed no matter which end of the pin you insert first.
Use your fingers to align the metal and plastic pieces of the safety sliders as closely as possible to their proper position in the housing so there's a clear circular hole all the way through where each pin fits. Align them to the best of your ability, then pull the trigger blade 2 or 3 times the whole time you're trying to insert the pin(s). If that alone doesn't do it for you, you could try inserting a small pin punch through one of the holes before you even start that process, then work on inserting the 'real' real trigger pin through the other hole. You may have to alternately bear down then release some pressure against the rear of the trigger assembly (i.e. toward the front of the rifle then toward the rear) while you're trying to insert the pins because the fit of the pins through the receiver and trigger is a pretty exact fit. However, it sounds like maybe you already have that part of the assembly process figured out.
If you try inserting the front pin first while working the trigger blade and it won't go in, try inserting the rear pin first. Then insert the other pin while working the trigger blade again if necessary.
In some instances I think I actually removed the trigger blade, then operated the sear the blade hits by hand, up and down--up and down, while inserting the pins. Then I reinstalled the trigger blade, spring, and pin.
I think there's a good chance that will get you somewhere. This is assuming you have that stamped piece of metal that fits inside the plastic piece inserted correctly. If the above doesn't work post again and I'll put my better thinking cap on because I'm working from some rather distant memories here. I worked on a ton of those triggers and I know the problem to which you're referring. Once you've figured it out and done it once you probably won't forget again.
Safe and Happy Shooting!
Ed Krzynowek, The Airgun Tune-Meister
"We can rebuild the squirrel. Make him stronger, faster...We have the technology"---Skyler M.