This is not necessarily the case, even if load balancing is configured to evenly distribute traffic between the SIM cards.
The thing to understand about load balancing is that it does not combine bandwidth available on both interfaces but rather, with an addition of a second SIM, it has additional resources to establish more connections. It means that a single connection will still use one mobile interface. For example, if you were downloading a large file from a single host, maximum download speed would not exceed what is available on the SIM the connection is made from, even if there is a second SIM and device operates in load balancing mode.
Another thing to take into account is the signal/connection quality. More traffic will be forwarded through the SIM, which has better signal characteristics and available bandwidth.
Best regards,