In term of port aggregation the use case it's different from what you are looking for.
Let's say your CPE does have 2 ETHs=100mbits but your radio link is capable of 300mbits, then you can limit this bottleneck by bonding both ETH1+ETH2 at CPE and connecting 2 cables to your router you can get almost 200mbits capacity on WAN and keep up with your radio link.
On the case that you are looking for, other than load balancing, you can see just for study purpose this lecture about VLAN bonding where you can aggregate traffic from different devices in your network. But this will imply adding switches and extra things, which I don't think is what you are interested into.
It's just for reading and learning.
https://wiki.mikrotik.com/wiki/Manual:C ... with_Bonds