Nest, Blue Wire, W1 and W2

The wire currently hooked to Y and C will be going off to your air conditioner condenser unit. Y is cooling.

C is common, and yes, you can connect another wire to that terminal.

You appear to have a two-stage furnace however, with W1 and W2. That means you have a couple options:

1 - Install another wire

To take full advantage of two-stage, you need to connect all 6 wires (R, C, Y, G, W1, W2).

You can run another physical wire, or buy 3rd party accessories that add a unit at each end that allows doubling up of some of the wires.

2 - Run as single-stage

You can possibly run your furnace with only W1. Many (all?) furnaces will automatically go to the second stage after they're run in single stage for a few minutes. This isn't as efficient as if your thermostat tells it to immediately go to second stage when needed, but is still usable.

3 - Run without fan control

You can potentially skip hooking up G (fan) but this means you can't run the fan in 'on' mode (or use periodic air circulation features) -- it'll only run when the furnace or A/C is on. (I'm not entirely certain about air conditioning with this setup though -- I am fairly sure A/C call will turn on the fan even without G connected, but not 100%, and it may vary by furnace model -- be sure to test if you are going to do this).