Well, medically speaking, there may be room to house multiple female genital apparati in the abdomen w/o much discomfort if they are staked vertically between the navel and the anus. Various medical conditions indicate there is some significant spare room in the abdominal cavity for more organs, human wombs aren't that bulky at rest, and twin pregnancies prove the human body can manage multiple pregnancies at once. Double penetration of multiple abdominal vaginas in all likelihood wouldn't be much different from double penetration of vagina and anus, and if anything significantly more comfortable, since both canals would be built for intercourse. Far-future genegineering capable of the mods we see in TiTS almost surely can make things easier by making liver, guts, etc. more efficient so they can afford to be smaller and create more room in the abdomen. So on second thoughts, prevailing artistic convention notwithstanding, in all likelihood the best (most plausible and manageable) arrangement of multiple pussies should be vertical stacking. Characters with this mod should have a different kind of undergartments or bodysuits that cover from the waist downard if they care to have their crotches covered.
More or less the same reasoning may be valid for multiple penises, although in such a case it may be best for comfort to use genegineering to house testes in the abdomen.