Drobo

can I do this - switch and two computers

Can I connect two MacPros and a drobo pro with this switch

http://www.bhphotovideo.com/c/product/464635-REG/Netgear_GS105NA_ProSafe_5_Port_Gigabit_Desktop.html

will both computers be able to see the drobo and use time machine to back up to the drobo?

no.

two computer cannot access a drobo simultaneously.

One computer could access it and then share that access via a network (set up drobo as a shared directory/drive just like you would share a USB drive across a network).

To do it without a computer you’d need a Drobo Elite and access via iSCSI - or a Drobo FS.