In parallel of course! I've actually found a way, will post a picture later. Basically, on Path 1, you split after the Amp block, have one of the IRs go out into Multi, while placing a Volume block (or anything with low DPS tbh) on the other split end of the path. Have the split path with the Volume block go into path B. Split path B, put 1 IR on each split path, merge the two paths together. Sounds great.
Hmm... I don't agree. Mixing single mic IRs of the same brand, let's say a V30 from inside the Friedman 4x12, is actually a very good way of getting a desired mix going on. Considering the fact that the built in mics in the Helix are few all and the fact that you have to Hi Cut them in order to get a somewhat realistic sound, doing the same thing with IRs is just better, since you don't have to bother with EQs and you have plenty of different mic options at your disposal. The pre-mixed IRs from ML Sound Lab and Ownhammer do sound amazing indeed, but what if you want to customize the mic placements yourself? Say you want to have a really dark SM57 and a really bright R121.
Let's say I want the following mics: a Mohave 200, a Shure SM7B and a Royer 121. Easy, I take the IRs, I audition the mic placements by themselves, then I blend them in to taste with the merge block. And no EQing needed whatsoever, meaning that you're 100% sure that you're not cutting "realistic" frequencies and that your virtual mic'd up speaker sounds just like a real mic'd up speaker. Because, IRs don't need any EQing, Hi/Lo Cutting or whatever to sound realistic, they are already the exact replica of a mic'd up speaker in a cab.
A more extreme example: let's say I want to add a room mic to the whole deal (the MK84 for instance) and that I also want to have the 7B and the 121 on the same speaker. Just as easy! You take a pre-made mix that combines the 7B and the 121 (meaning that there are the little imperfections of both of the mics on the same speaker, something that a Cab block or 2 IR blocks can't reproduce), put the Mohave on another IR and then to top it off grab a "room mic" IR. Lots of options.
Something more subtle would be to take a pre-mixed IR that you really like, let's say the Modern mix from Ownhammer that has the SM57 and the MD421, and you want to embelish that mix with some expensive mics :D So you take a Neumann U67 and maybe a Room placed MK84 or something like that to beef up and sparkle up the already great mix to begin with. Etc. etc.
Soooo many options with 3 IR blocks, it's ridiculous.