Note that important part of forming two-party systems are single-member districts. It seems unlikely that just removing FPTP and keeping single-member districts changes this in a significant way.
A good example of a proportional (and thus multi-party) system that uses single-member districts is MMP[1]. You could argue, though, that this system doesn't fully keep single-member districts because some of the representatives are chosen from a national or regional list and such lists are effectively giant districts.
There is still much to be said in support of non-FPTP single-member systems, though, since they allow factions within a party to break off and present their own policies. Even if such a splinter party never replaced the party it split from, it could still prevent the bigger party from becoming too complacent and reliant on being the lesser of two evils.
