Can I use two ADSL modems at once?

No. ADSL is an extension of the technology used for providing normal telephony services, and as such cannot cope with two conversations of the same type on the same line at the same time e.g. two conversations on the phone, two ADSL sessions etc.

You can use an ADSL modem at the same time as a normal analogue modem (e.g. a 56K modem on the analogue part of your phone line) but you cannot connect multiple ADSL modems for use at the same time on the same ADSL circuit.