This probably cannot be done in the way you mean.
I take your question to mean that you want to redirect high-bandwidth uploads to a particular ISP, and leave the other link free for other things.
If both links are to the same ISP and you can use the same public IP address on both of them, then this is possible.
If the two connections aren't using NAT and can be routed either/or, then sending all upstream traffic over link A and receiving all downstream traffic over link B is pretty straightforward - simply tweak the IP routes at both ends to favor the appropriate link at each end, and have the other link configured as a backup route (higher administrative distance value)
I'm 99.999% certain though, that you either have two different ISPs or else two links to the same ISP, but with different IP addresses on them.
This means that you cannot achieve what you want. Even though it's quite common to talk about Internet connections as "uploads" or "downloads", the reality is that all connections are bi-directional. When TCP makes a connection to some host, it doesn't know what is going to happen across this connection. It doesn't know if a user plans to upload a video to YouTube or download the updates to their entire Steam catalog. As such, the connection can be routed based on several criteria, some of which involve the current load of each ISP connection, but once the connection is routed through a certain ISP, then it must STAY through that ISP (actually, it must stay through the same IP address) and so if it turns out that the majority of the data segments are being sent in the "upload" direction, and the connection was made across the "download" ISP, then that's just too bad because you can't switch the connection over to a different IP address on your end (not in the router anyway - because TCP is end-to-end), so you can't swing the "upload" traffic onto the "upload" ISP.
It gets even more nuanced than this, because every TCP connection sends lots of upstream segments - when downloading, every few blocks of data requires the recipient to send an acknowledgement that the data arrived successfully. These ACK messages are small, but they do create an stream of "upload" data even during a download. You would have to be careful to know the difference between the ACKs of a download connection vs. the larger utilization of an upload.
When given a spoon,
you should not cling to your fork.
The soup will get cold.