Multiple IP networks on a single L2?

I have always followed the unofficial best practice of putting only one L3 net (IP network) on a L2 network. If I added a network, I added a VLAN.

However, technically there is requirement for this: one can not just assign multiple IP addresses to an interface but the IPs can have different prefixes (and hence constitute different IP networks).

I am considering the following scenario: I have two sub nets routed over two different providers. I have some machines which should be accessible on both addresses. So I am considering creating a single VLAN called “WAN” that includes both networks. A node on this VLAN can then add either an IP from ISP1, an IP from ISP2 or both, depending on requirements.

Is there anything wrong with this? Should I ALWAYS create a separate L2 network for each IP network?