HI,
As your query 1) a. GUP will receive update from only designated SEPM on which it configured as GUP and reporting to that SEPM. b. Which system will act as GUP, this decides from SEPM on which GUP IP configured to receive updates and distribute to local subnet. c. It decides from reporting SEPM on which GUP IP configured.
Query 2) As you mentioned some GUP closer to SEPM and you want some GUP's report to that SEPM and rest GUP to other SEPM's. Here if you have WAN connectivity, through MPLS or Leased line , geographical distance does not matter whether its near of far ur nearest SEPM, its depend on the bandwidth between ur branch office GUP and ur SEPM site, So you can point ur half GUP to any SEPM which bandwidth is appropriate,
As you mentioned you have 20 GUP's at ur branch offices reporting to 2 Site's SEPM and all connected to each other through WAN. As designed architecture of SEPM for infrastructure you can connect all GUP systems with single SEPM at any one site for better managing and administering your all branch offices systems, if you have total number of systems less than 50000 for 11.0 version SEPM and up to 80,000 if you have 12.0 Version SEPM, No need to deploy separate SEPM at your other site if total number of systems not more than as mentioned above for better manage centrally.
I will appreciate and welcome for any comment or changes on my post here by other SEPM Admins and forum experts for their valuable feedback.