First of all, I haven't the foggiest idea when it comes to modding, so apologies if it seems silly.

I've been using this pretty heavily in my game now as it's a lot of fun seeing how busy it gets on the map,
but would it be possible and/or difficult to add an option for more than 1 resource being requested at the request depot? Say two resources instead of the one?

That being said, I can understand from a balance point of view, one of these being able to request 2 resources is
probably too much of an imbalance, maybe halve it's storage size in that case, as it's storing two different resources

Why not just put 2 request depots next to each other? They're not expensive to build.

I'm just building small sub-factories for special purposes, eg one row of assemblers to make green circuits, and another to make red circuits. it is easy to set them up with separate requesters (as Andrew has said), but with their size of 3x3 tiles (and i understand and see the advantages of square footprints) this looks quite complicated and space consuming at first. they require 4x3 or 5x3 tiles along my "main road", or a small additional "side road" that can service those 4 or 5 depots. a truck/drone based transport system just can't be as small and specific and universal at the same time, as eg bots with their overpowered 1x1 chests :-)

in the end, it's no big problem and only a small additional challenge to find a solution once and use that template for many similar sub-factories. i just finished two versions, one of them being the following :
3 tiles for a row of beacons, 3 for first row of assemblers, 1 tile for inserters, 3 tiles for (optionally weaved) belts (2 to 8 resource lanes "in" and 1 to 4 product lanes "out"), another tile of inserters and another 3 tiles for a second row of assemblers (3 more tiles for beacons already belong to the next block). total of 14 tiles width, leaving exactly 1 tile for supplying the beacons with power, followed by the next 15 wide tileable (beacons overlapping) building block, that can be extended (made longer) while belts can support throughput (yes, i use belts, but only inside each block, not to connect blocks with each other) ...

if you can't imagine this yourself, the following are two such blocks, one for red+green and one for grey science, total width of 2x15(+3) tiles, fitting in 0.5x2 chunks per block. currently still unbeaconed and only with assembler2 and yellow belts, but should already be good enough for 90 spm, speedbeaconed and with assembler3 probably 750+ spm :-) what more could you want ? :-) LOL
https://kirkmcdonald.github.io/calc.html#zip=dY5BDoMwDAR/kxORQltBBcpjrOAiq3GMiDnwezj10CSStRfvjmYBBe9sP9rBGabkX4YUOXs4VBiUJNkcCFNAu0H4Tp+pd12UlbJSKF6PjimSwn6WK0Nrkh1b5Dq0zjMsyxGxbZmfv5vvfDeMi1rd/r92AQ==

While it does make sense to have a requester be able to request more than one type of item, the mod works by using recipes, and the requester depot is an assembly machine. That means each request option is a new recipe for the game.
For requesting a single item, one new recipe per item is created. If you were to request two items, each possible item pair would need its own recipe. That would suck.

And since this is the very foundation of how the mod works, changing that behavior means essentially writing a whole new mod.

So, probably not gonna happen.
But you can use the advises from the other comments and use more than one requester. I've also did it in my game, so I now have a blueprint for the requesters and their outputs into an assembly line for the various possible recipe combinations (1 ingredient, 2, 3, 4, 3 + fluid, etc...)