Will I die if I climb doubles with an 8.7 and a 7.3.

I don't really want to buy two ropes for purely double use. I can get a deal on a Mammut serenity 8.7 and beal gulley 7.3. Figured that would maximize the capability of my quiver. I've heard they recommend using the same rope or at least ropes with the same stats but why not? Thanks for the advice.

ive used my 9.5mm infinity with my 8mm phoenix ... and various other combinations

a potentially major issue is the belay device ... you really need to test it to make sure it can handle both rope diameters simultaneously ... a device might not be compatible with both diameters, and even if it is in theory, in practicality it might be hard to catch a fall especially if mostly loaded on the thin strand

unless you really need the double rope feature, it might be better to use the single and tag the half ...

or mostly clip the single and only clip the half for the occasional wandering piece and traverse

of course if you have two followers the difference in rope diameter, on lead belay anyways, is moot as you can get each follower to belay a strand

;)