I have a kegerator made for one keg. However it holds 2 so I like to have one near ready when the primary keg goes empty. My question is. If I transfer my latest beer batch from the secondary to my standby keg and pump it up with CO2 do I have to leave it connected to the co2 line? Will it still carbonate or at least start?
You need to keep it hooked up typically.
Think about it this way, you have 4.5 gallons of beer, that is supposed to be carbonated at 12psi. If you have 0.5 gallons of headspace, then the CO2 pressure is reduced 10 times as it expands into the 5 gallons, so the resulting beer would have a carbonation equivalent to 1.2psi.
What you can do is repeatedly pressurize the keg, say, every day, but not keep it hooked up permanently. After a couple of weeks the keg will be carbonated.
Another approach is to force carbonate by setting the desired target pressure and rolling the keg around back and forth until the CO2 stops flowing. If the keg is chilled, this can take less than 15 minutes.
You do, but you're able to, at least if you time it right.
Let's say – as you assume – your primary/dispense keg is already stable; it's carbonated and you have a batch waiting in secondary, and you intend to pipeline new kegs through the kegerator. What I would do is … when you're about 2/3rds of the way through the first keg, give the headspace a couple/few more psi than serving pressure, then switch over to force-carb the new keg. The huge volume of headspace pressure relative to the already-carbonated beer will let you serve the last third without issue (there's enough residual pressure in 2/3rds of the keg's headspace to draft for many many pours), balanced by enough time to force-carb the new keg at the appropriate volume/time for the temp.
To provide a bit of contrast, consider the case where you start off with two totally full kegs in there. You're force-carbing "A" to start off; done after a week-ish. You start to dispense from "A", and as soon as you do, the now much-smaller headspace needs to be replenished. After that occurs (maybe just a few hours), you can switch carbonation over to "B", and start it's "week long" journey to carbonation. You dispense from "A", now it needs a top-off. Few seconds, then back to "B" to continue force-carb. Dispense from "A", top-off for a couple few seconds, then back to "B". It keeps going like this, with the periods between ever longer each time.
The key thing is that you have a second slot so you can get the beer down to temp and force-carb it for at least a week, which is (hopefully) long enough to last a 1/3rd of a keg … in that way you should be able to pipeline kegs through the system pretty well.
You can do this by switching the gas between the two kegs, but it's far easier to get a gas manifold or y-splitter and supply gas to both kegs simultaneously.