Yes, you can have multiple MPs and DPs (with PXE) in untrusted forest. I've more than 3 MPs + DPs (with PXE) in untrusted forest without any problem.
1. Use SQL connection account to connect to DB from remote MP.
2. Select Require the Site server to Initiate Connection to this Site System option. More details
here.
3. AD forest discovery account must have access to publish MP into system management container (untrusted forest). More tips
here.
4. You can also perform AD Discovery on untrusted forest more details
here.
5. If you've multiple untrusted forest under primary then take care of network access account configuration. More Details here. This is not required in CM 12 R2. In R2 we can have more than one network access accounts. More details
here.
6. If you are looking to create TS boot media from untrusted forest DP. More tips
here.
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