While the regulations clearly state that an issuer can't conduct a Reg CF offering on multiple funding portals simultaneously, is there anything that would prevent a potential issuer from running a Testing the Waters campaign on multiple funding portals at the same time?

Nothing in the rules prevents it although platforms could impose their own contractual requirements (eg, if you use our platform for TTW you have to use us for the actual offering).

BTW, the SEC were not actually expecting that TTW would happen on platforms! They thought TTW would be on social media or on issuers' own sites.