In the below, 1.1.1.0/24 represents customer A and 1.1.2.0/24 represents customer B.
In the forward chain, permit traffic from A to B and from B to A, then drop everything else sourced from either network. In the input chain, drop all traffic from them.
/ip firewall filter
add chain=forward src-address=1.1.1.0/24 dst-address=1.1.2.0/24 action=accept
add chain=forward src-address=1.1.2.0/24 dst-address=1.1.1.0/24 action=accept
add chain=forward src-address=1.1.1.0/24 action=drop
add chain=forward src-address=1.1.2.0/24 action=drop
add chain=input src-address=1.1.1.0/24 action=drop
add chain=input src-address=1.1.2.0/24 action=drop
This can be simplified with address lists if you feel like it:
/ip firewall address-list
add list=CustomerToCustomer address=1.1.1.0/24
add list=CustomerToCustomer address=1.1.2.0/24
/ip firewall filter
add chain=forward src-address-list=CustomerToCustomer dst-address-list=CustomerToCustomer action=accept
add chain=forward src-address-list=CustomerToCustomer action=drop
add chain=input src-address-list=CustomerToCustomer
Make sure you place the rules at a sensible place in your ruleset.