Factory external selectable trigger groups are going for $1200 and above if you find a nice one. Buying an extra top first trigger is a sensible solution, so your MX-15 will not be inflated with the trigger price when you want to sell it.
Dino also wants a trigger he can switch over to use on his new set of O/Us too. But still, his answer is to buy an extra trigger, top first.
Dino. Your current bottom first is worht about 700. And the real problem yo have is actually finding a EXT in good used condiotn. A lot out on the ranges and safes, but no one wants to sell theirs to you. (or me either)
you don't need the externally selective trigger. They are usefull for sporting when you want to switch bbls. for different stands, but in your case, you are going to use either top or bottom first, but not both. Have your current trigger fitted with the internally selectible "tree". A lot of the gunsmiths on this site can do this for you. Much cheaper than another trigger!