It’ more complicated.
mPCIe / m.2 are only a socket who is connected to some BUS of speed and many answers from MikroTik say about a USB BUS as limit factor.
USB EndPoint Limit cannot give you install two “profesional” cards, USB 3.0 port is used OR mPCIe slot…
Please see:
RBM33G - mPCIe ports – http://forum.mikrotik.com/t/rbm33g-mpcie-ports/135927/1
Tips: LtAP (Not mini) & RB33G EndPoints Limits – http://forum.mikrotik.com/t/tips-ltap-not-mini/141186/1
If your high LTE Category modems will work both, and your ISP not limit you a speed then maybe you can receive max ~800Mbps with decent scenario.
In Poland every ISP at every BTS/Tower limit user speed and you have differ speed at night/day profile.