Electric set up for multimotor model

One battery for inboards, one for outboards is ok.
You need a speedo for EACH motor = 4 speedos. (so you'll need three Y leads to connect them all to the motor/throttle plughole in the rx. Disconnect three of the four RED wires, or you will have four bec's fighting! Keep the yellow and black connected. black is common return, yellow is the data signal for the speedos. Don't be too mean with the current rating, bigger is better, up to 50% more than the motor demands.
100 watts - pound is ok.
Lipo's - more S = more volts. The on-load volts depends on the amps being drawn, and as you are having two motors on each lipo, that's twice the current.
The current drawn depends on the prop diameter and pitch, so you need a WATTMETER to see what is going on! Don't guesstimate, you will be wrong, and could fry a motor or speedo.
The kva number on a motor tells how fast it will turn for each volt of supply. (ie a 10 kva motor will turn ten times a minute for 1 volt of supply, not that you could buy one that slow!)

True, Quorneng, but it helps the model's flying behaviour if the mass can be kept in the centre. Four lipos would require four lot of wires, or putting the lipos in the nacelles, which is moving the mass outboard, and has the effect of slowing down roll response.

Dean
Because you are juggling four variables, you cannot guesstimate the duration. You need to know how many amps the motor is pulling, and that depends on the pitch and diameter of the prop. (as well as the brand!). When you know that, you can look at the lipo capacity, and get an idea of how long it will deliver that many amps.
Don't forget that the receiver, the speedo, and the servos all use a bit of that power, albeit a small amount, (unless you have Klippy's digital servos!), and that is being taken from the lipo that has the bec connected to the rx, so that one will fade first. (the other three speedos are fed from the lipo connected to it)
THEN you can mathematically say - this combi will give me x minutes. But as we never, ever run lipos to the death, reduce that figure by 25%. (or thereabouts)

Don't forget how electric motors work, they see a number of volts, and decide they have to go at x rpm's, so suck more and more amps until they do. (or the windings, the speedo or the lipo gives up!) No matter what you do, the motor will not go faster per volt than it is designed for, and if you load it up with a monster prop, it will bust itself trying to spin it at the same speed.

Quorneng - you could probably get away with a common negative from each side, but you'd need separate positive wires to each speedo, if you put those in the nacelles. The weight wouldn't be huge, but it's going to be a heavy model to start with, and every bit of weight saved is a bit more lift available.
If you keep the lipo - speedo wires short, with the speedos in the fuz as well, then you have six wires down each wing, plus ailerons, etc.
If you are going to have a stack of four lipos in the fuz, it's simpler - physically to have two bigger ones. (There are also less connectors to get crossed over!)

BB, as you have lost the two-wire speedos, you'd be better off scrapping the can motors, and fitting brushless ones, which are much more powerful for the same can size.
It is doubtful whether you could even FIND four matched two-wire speedos, now!