Can I buy 2 units. The 1st unit is GSM modem Type SGDK, and the 2nd unit is Type E. Then I can use both units with the same development kit belonging to the 1st unit?

No. You cannot use MOBITEK SMS Gateway Development Kit with Type E Modem.

We are selling you the modem and not the licence of SMS Gateway Development Kit.

Should you wish to use the SMS Gateway Development Kit, then you must use Type SGDK Modem.