I built meraki device as below topology.
It is a warm spare setting not stacking.
After that I saw many looping issue, so all of L2 MS were down.
Should I set port-channel on the each L2 MS ?
Because you have dual connected everything so there are several loops. Spanning tree will block the redundant links, so it should still work fine.
What version of MS firmware version are you running?
I'd attempt to re-build this first, getting both MS HA switches online and checking into dashboard correctly.
Firstly, factory reset. Power up the 1st switch, make sure you set it to be root bridge. Power up the Second switch, getting it to be the backup. Then attempt to plug in primary up-links. Then plug in secondary up-links.