how to conbine downLink with an upLink

Suppose that i have :
1 - first link with 512 kb tx / 64 kb rx <---- uplink
2 - 2nd link with 16 kb tx / 1 mb rx <---- downlink

I was conbining this two link using windows server with isa server is this possible with Mikrotik RouterOS(do i have suppose necessary to have two public Ip or not )
and what is the Condition ??
or it’s up to the downlink ISP to configure this ??