Can I use both the M.2 Key B slot and the Mini PCIe slot at the same time?

No, the M.2 Key B (J6) and Mini PCIe (J5) cannot be used at the same time — only one can be used at a time. When using the M.2 Key B module, set SIM_MUX_SEL to high level (disconnect Pin 1 and Pin 2 jumper cap of J8), and when using the Mini PCIe 4G module, set SIM_MUX_SEL to low level. (Page 19-20)