First question to probably ask would be: Speed to where?
The ports are connected to a switch most likely, does that switch have any ports that can handle speeds larger then 1GB/s?
And is the device you want the added speed to connected directly to that port, or is it connected to other switches in a chain that all have that increased speed-possibility?
The entire path needs to allow the higher speed for you to have any benifit of it for traffic to 1 device, if not, you'll only increase the complexity without any profit. The only thing you might want to do is add stability by setting up the ports as a failover-bond to deal with 1 port being unplugged.
For traffic to 2 (or more) devices at the same time at 1GB/s speeds, there are a couple of routes you could take, the "quick and dirty" method would be to add a second network card to your VM, seperate IP's, and set rules to use one or the other for traffic to the specific devices you need the increased speed to.
If all further hops and the end-device can handle the 1Gb/s+ of traffic, or it goes to multiple devices and 1GB/s max per end-device is fine, the basic setup would be:
Create a Linux bond, select the mode [1] (which depends on your switch which one you should take, and can sometimes be a little trial-and-error), add the 2 network-interfaces to it, create a/change the Linux bridge, set the bridge port as the bond you just created, then sat that bridge as the network-adaptor for your VM. Also, this is always for all the VM's on that bridge combined, not just one.
[1]
https://pve.proxmox.com/wiki/Network_Configuration#sysadmin_network_bond
