If your ISP is not allowing MLPPP (which is fairly common), then you have 2 options.
1) Load balancing: This can be done by multi WAN routers. This will provide some reliability and some capacity increase, but you will not get increased speed.
2) Broadband Bonding: This is a new technology that enables bonding of Internet lines (not only DSL) from any service provider by using layer 4 algorithms. You can google the term “broadband bonding” for more information about the solutions in the market.