Hi!
First, you should check your SCSI card model because some models of the B&W G3 were sold with a Ultra2Wide SCSI card using LVD (Low voltage differential) and the first revision of this card was buggy when using external peripherals.
For this reason, my father had to buy a 2930 to use its MO drive ...
Then, depending of the SCSI protocol used, you can connect up to 7 or 15 peripherals on 1 card. Each peripheral had to be identified using jumpers, dip switches or rotary selector (depending of the manufacturer).
This is done by chaining peripherals so if your sampler does not provide a second scsi connector, you will not be able to chain it and you will have to connect it as the last one in the chain.
Then, the difference between 2930 and 2906 is mainly the external connector used (also performances I think but here it is not the goal I think).
About models useable on this computer, you should be able to use 2906, 2930, 2940 but also 29160 etc. Main difference between Mac and PC cards are the firmware which allows you to boot from SCSI drive or not (I may be wrong, it's a long time I did not "play" with them).
Of course, you can install as much card as you have PCI slot available.
Finally, I checked your sampler model on google and saw that there is only 1 SCSI port and no ID selector so its ID should be static and you will have to check it in your sampler manual.
Then, yes, 2906 should be a good option to add another sampler. Sad to see that they did not put another port to chain them.