here we go again.....
Rem, Firstly, welcome to ribnet.
You have opened with one of the more controversial discussions on this forum, so expect some strong opinions!
There is no generic answer...... Do a search for "twins", there is a good afternoon's reading out there. Also be aware that comments relevant to umpteen hundred HP rigs are not necessarily valid at the "100-ish total" ballpark.
A high level summary:-
beware the generic "nn% useable HP" figure..... with smaller set-ups the top speed is less and the bits being dragged through the water are usually smaller. (therefore the absolute drag for the rig is substantially less proportionally to the Hp available to overcome it)
The powerhead may be shared with another HP, and the leg / gearbox may be shared with 5 or 6 other engine sizes! e.g. my old Yam 55 I got spares for the gearbox end off a 90! - So how overkill was the gearbox on the 55? - to put it in perspective, the 'box & leg on my 60 is about the same cross section as the bottom end on my Suz DT25 So when I swapped engines I got more gee gees & less drag.....
If you are looking at 60 or 70 HP, most are still emergency pull start-able, and will likely run without the aid of a battery once going. (beware some of the new machines tho' -some (Optis?) need a healthy battery or it's no go.
Weight wise you have to look at individual combinations (I have posted examples in the past) - and look at different manufacturers too. Also as said above, don't forgret to add the deadweight of the aux.
What may be the deciding factor if your hull is secondhand tho' is the inevitable cut - out for a single. I nearly went for twin 30s when the 55 died, , but the woodwork involved to fill the transom cut was going to be too much hassle so I am still running single + aux.
Search the forums, and have a read with an open mind. One proverbial man' s meat & the other's poison is a phrase that springs to mind here!