Dear Mr. AppleJaxJap (Electrical)(OP)21 Apr 22 01:32
" I have 2 non-utility transformers (480-415/240) that are solidly grounded in 2 different buildings.....I want to feed the 3phase 4 wire load with the transformer through the ATS, #1. Obviously. There are no GFI concerns.... #2. Do I need a 4 pole ATS?... #3. Looking for code or IEEE info".
#1. It is essential to treat the N1 and N2 separately in order to prevent circulating current. However, the G1 and G2 can be connected together if feasible. Threat each transformer as an independent four wire + G power source.
#2. Use a 4-pole ATS to avoid neutral circulating current, including simplifies individual ground fault protection. There are schemes showing use of three-pole ATS with alternatives ground fault protection etc. Please avoid it. There are 4-pole ATS available at very little additional cost.
#3. In US NEC is the Law. Comply it. Any mention in IEEE are "recommendations ", as a good reference.
Che Kuan Yau (Singapore)

Basically, you can have only one bond between grounded circuit conductors (neutrals) and ground. If you have a bonding jumper at each transformer, you have to use a 4-pole ATS to avoid two ground bonds. You could have a bonding jumper on only one of the transformers, with the neutral not switches by the ATS (3-pole), but this can result in some long ground fault paths when the source is the transformer without the bonding jumper. I would agree with Che to avoid this.