Can I to use 2 adsl lines simultaneously?

I have done this before with 2 4MB ADSL lines, however have never been able to get them to bond. PFSense is a BSD based firewall router that has built in support for multiple ADSL lines in a load balanced environment.

There is also various dual ADSL routers available. These will provide you similar solutions. Molly already provided a link to one that can. I have not been able to source these locally in ZA but know they should be available in the States with no issues.

Yes, you'll need a dual ADSL router for bonding the two connections.

Zyxel's P-663H-51 has two ADSL2/2+ WAN ports. With the ATM-based multi-paired bonding feature, the two ports can be grouped into a logical link boasting the bandwidth twice as fast.