From what I'm reading the motors for the vacuum fans on the new JD planters have case drains. I am not sure if that allows you to plumb them in series or not.

On the older planters that I've dealt with (JD 7200 and JD 1770, both 16-30) we had 2 vacuum fans on each of the planters. Neither of these planters had case drains on the motors that drove the vacuum fans. In that situation you are not supposed to have back pressure on the return line of the motor that drives the vacuum fan. If you were to plumb these two motors in series it would result in a lot of back pressure on the return line of the first motor.

Edit: the following paragraph is just a description of a way to plumb vacuum motors in parallel

The solution to this (if you want to run them off of one SCV) is to have have a T in the pressure line and a an adjustable restriction on each of the two resulting lines. The return lines then each come together at a different T and then run to the return on the tractor. For whatever reason one fan or another may require a different pressure than the other fan to pull the desired vacuum. To even this out you use the adjustable restriction (a needle valve for example) on each line as required.

As I mentioned, perhaps it's ok to run motors with a significant amount of back pressure if they have a case drain and this would allow you to do as you mentioned. If not though, like nwb soky said, the only way to avoid getting in and out of the cab to even out the vacuum on different fans is to plumb them to different SCV's and control the flow from the cab.