Look at the codex carefully. Buying two AUTOMATICALLY twin links the weapon. Note cost for plasma rifle as +16/+24.
Twin linking is an excuse to mount target locks or drone controllers.
Normally if you have two different weapons you get a multitracker. Try give it room on the suit, it cost more hardwired.
My favourite alternative is to place two incompatible weapons on the suit like fusion blaster and flamer. Fusion blaster kills tanks and power armoured troops, flamer kills light infantry, so you really only need one tracking system as you only need to fire one system at once (ok you miss out on the chance for the fusion blaster to kill one extra very very crispy light infantryman). So instead of mounting multitrackers mount drones or target locks and resign yourself to using one suit weapon.
An example
Shas'vre: Fusion blaster, flamer, drone controller and 2 gun drones, hw target lock
Shas'ui: Fusion blaster, flamer, drone controller and 2 gun drones
Shas'ui: Fusion blaster, burst cannon, target lock
Suit two ALWAYS fires first, establishing the team target. The team leader and suit three are then both free to target seperately. This team is geared for close assault, the target lock and fusion blaster is primary loadout so three seperate anti tank shots can be got on three seperate targets if consistantly lucky (not unforseeable if they hit). Now I could have twin-linked fusion blasters for a better chance, but this would mean an expensive and overdedicated unit, it costs no more to take a flamer instead. So I mixed in some a secondary role. The burst cannon, flamers and four drones give respectable anti-infantry fire, at similar ranges of engagement as the fusion blasters. I lose three anti-tank rerolls in return for a whole new role. If the army has no tanks left or any to begin with the anti-infantry firepower is sufficient that the team can perform normally with only a small drop of efficiency.
What I like best about this team is that when firing on an APC a quick kill (which will most likely happen if I hit) will allow the other team members to turn their firepower on the troops scrambling out of the wreck. If one suit fails to kill the tank the others can still maintain focus on the primary target.