Hey you have the logic backwards.
The ASN prepending happens from Meraki to the DC router.
So if a certain spoke has hub1 as first prio it will take that path for a certain subnet.
However the spoke network will be advertised via eBGP to both DC's but DC-2 will have a second ASN prepended.
Of course this is only important if you have duplicate IP space in both DC's.
So if you would have a scenario that a spoke sends traffic to dc hub 1, but the end host is at dc 2 then that traffic will have to be tromboned using the DC interconnect that would have to be in place. In that case return traffic will also first return to DC-1 before being received by the dc 1 hub for encapsulation in SD-WAN.