Welcome, Dave, to the OEM forum.
Let’s get this out of the way first - we all had to start learning somewhere once upon a time, as long as you’re prepared to give it a go, you’ll find most people here are willing to share their knowledge. The second part there is there’s only one dumb question: it’s the one where you assume we know what you’re thinking and what you’re looking at. As long as you tell us what you have and what you want, you’ll be fine.
The emonVs is both a 5 V d.c. power supply to power the emonTx4, as well as a voltage reference.
It enables the emonTx4 to accurately measure the voltage and calculate the real power (which is what your supplier’s electricity meter charges you for). Without that, you can end up with a rather inaccurate guess at what you’re consuming - and inaccurate for two reasons: first, the voltage isn’t constant but varies according to the time of day and what everyone in the country, and particularly in your locality, is doing; and secondly, watts = volts × amps is only true for kettles and immersion heaters and not much else. A microwave oven on standby can, using those maths, appear to consume something approaching 50 W of apparent power, when the real power, as measured by your meter and emonTx4 with the emonVs, is 1 W or less.
If both supplies come into your house close(ish) together, and the consumer units are close(ish) together (a couple of yards), or you can run cables between the two, you could get away with a single emonVs and a single emonTx4, but my guess is this won’t be the case.
This leads to the next question: are the two supplies on the same phase or are they different? The way to tell (unless it’s clearly marked - unlikely) is to measure the voltage between the two line (brown) conductors, or the right-hand pin of a 13 A socket. Be very careful. If it’s truly zero, be wary, because I’d always expect either the full 415 V or thereabouts, or one or two volts. If you see 415 V, they are different phases, and in this case you really don’t want sockets on different phases close together, for obvious reasons.
Assuming your incomers, meters and consumer units are inconveniently separated, then unfortunately yes, I do think you need an emonBase and two sets of emonTx4, emonVs and a single c.t. And yes, you can add (and mix and match) c.t’s afterwards.
Do you actually need a 100 A c.t. on your incomer? Do you have a realistic idea of the maximum current you take? If you have two supplies, that’s a total maximum of 48 kW, which is a lot of power. It might pay you to go round and add up the worst-case set of appliances that you can envisage all being on at the same time, and see what the total comes to. But check the diameter of the cable the c.t. is to go around - it might be you need the 100 A one because of this.
[So far this evening, my voltage has swung between 230.3 V and 238.4 V.]