If you want to ensure that your network remains operational even if one switch in Stack B fails, it would be better to take 1 port from each of the 2 switches in Stack B to 2 ports on the core switch (Stack A). This way, if one switch in Stack B fails, the other switch can still maintain the connection.
If your switches support link aggregation (also known as port trunking or teaming), you could potentially increase the throughput between the stacks by using multiple ports. In this case, either option could work, but you’d need to ensure that the switches are properly configured to take advantage of this feature.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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