So, after entering the Hidden Menu,
I went to Field Test > Modem Settings > Network Mode
THere's a long list of bands that the phone supports, and you can click on each to test. It will show SUCCESS/FAIL
Below are the SUCCESS selections for both sims
SIM 1 (Singtel), WCDMA 850, 900, 1900, 2100 and GSM 850, 900, 1800, 1900
SIM 2 (Maxis), only GSM 850, 900, 1800, 1900
Now, if I switched SIM 1 to any of the GSM bands, the SIM 2 became able to select the WCDMA bands...
Which proves that SIM1/SIM2 can only exist in a (4G or 3G) / 2G combination or 2G / (4G or 3G)
FYI, WCDMA are the 3G, 3.5G (HSDPA), 3.75G (HSPA+) network types; and
GSM are the 2G network types
LTE is LTE ...