I've been digging into this... there are comments on a Doctor of Credit post that indicate you can activate many cards on the same account. The Instacart+ membership will extend for the months each card gives you. Then you can order using any of the cards you've activated and the correct credit will trigger.

I just tried adding P2's CSP to my account. The membership expiration did, indeed, extend by 6 months beyond my original CSR's expiration date. I think this is the way it's supposed to work, since there is now a hover message that clearly states each card that was activated and how they add up to the new expiration date.

I order from Instacart every week so I'll have some DPs to share if the credits actually trigger. But the DoC people think it'll work fine, as long as you actually take the step of activating Instacart+ on each card.