How the sound comes out of the rack shouldn't really be an issue - you can just plug the output of whichever module is last in your chain (in your case, probably the mixer) into whatever you're using - a PA system if you're doing a live performance, PC-style speaks for noodling around at home, etc. The modular's native level is a fair bit hotter than what people call "line" level, so you may have to turn down the volume a bit, but it normally works well. Buying a module specifically for "output" is seldom necessary.
I wouldn't buy two Mother-32s for a new rack - I'd rather have more variety - but if you already own them, okay.
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