I'm not an expert or anything, but I'm pretty sure that the two dads no mom thing is more likely to be discovered first since I read an article discussing the theory behind it (also disscussed the theory behind two moms which might be easier since you'd already have an egg instead of needing a third donor). Merging two sperm into one sperm with only 23 chromosomes would actually be very difficult because you'd have to get rid of half of the genes (and exactly the right half to make sure you have one of each and not two 15ths and no 3rd or something), but emptying an ovum entirely is less difficult (we already know how to do that from cloning experiments with animals).