You need to configure multilink ppp on both ends. This means in effect you cannot use 2 differnt ISP on the far end since the data must be recombined back together and the 2 isp's do not talk to each other.
It gets very complex when you try to do this without a routing protocol. The only way to truely do this is by running BGP between your ISP's but since you are using ADSL it is unlikely this is a option.
It is hard to even begin to discuss all the issues. If you look at other posts in this forum you will see many discussions of design using multiple DSL connections.
Your main problems will be that you cannot load balance the incoming traffic and you more than likely have different IP pools from the ISP's and the NAT issues that causes.
