Yes it is possible when used in combination with router's Load balancing functionality. Router's configuration should look like this:
1. Navigate to "Network -> Mobile" menu and configure your primary APN.
2. In "Network -> Mobile -> Multi APN" menu add secondary APN.
3. In "Network -> WAN" menu select "Load Balancing" option instead of "WAN Failover". Then make sure that "checkbox'es" are selected for both mobile interfaces.
4. Navigate to "Network -> Load Balancing" menu and create two new "Policies" and two new "Rules" (Existing rules can be deleted). Here is the picture how load balancing should be configured:
- Parameters, highlighted in blue are mandatory. It is essential to have two Policies on different mobile interfaces with "Ratio: 1".
- Parameters, highlighted in green can be renamed to your needs. You can enter any custom names of Policies/Rules, as long as you will select correct "Policy" for each "Rule".
- Parameters, highlighted in red are conditions, which should be used to differentiate the traffic, so that it would be routed through different mobile interfaces.
The idea of Load balancing: When new packet should be sent from the router, router checks Load balancing "Rules" from top to bottom. It if finds matching rule (which satisfies the condition), it sends packet through the interface, which is assigned to Policy of this Rule.
Because of that, special rules (which applies to only certain conditions) should be on top, while "default action" rule should be the bottom one.