if you hold with blue sentinel do you get 2 power?

Yes — you add 2 power total at the start of your next Main Phase.

Blue Sentinel's passive ("Your hold effects for holding here trigger an additional time") doesn't exclude its own "When I hold" ability, so that ability triggers twice when it holds a battlefield. Each trigger adds 1 any-domain power, for 2 total (per FAQ #10155). The power is added automatically — no exhaustion required (per FAQ #9539).