Generally speaking, BGP is not available from DSL providers, so true multihoming is out of the question. What you do have to deal with is having two sets of IP addresses, one for each ISP and useable only for traffic carried by the link provided by that ISP.
To get around the limitations, there are various hacks for different applications which you may or may not find acceptable. These include (in no particular order):
1- Outsource your servers to a well connected service provider so that link failures will not affect the ability of people to buy from you.
2- Multiple MX records (one per ISP) for mail. This may require running multiple mail servers as not all support multiple IP addresses correctly.
3- Hosting your own DNS servers, one per ISP, each delivering only IP addresses serviced by that IP.
4- Ping based outbound routing (look at the Symantec 200R, but do some research before buying :-)
5- Floating static routes and NAT for sorting outbound traffic (see the example in Chapter 8 of my book, and recognize that there is a high probability that when the DSL links fails, it will become a black hole... but you could put important traffic on the T1 and back it up with the DSL).
Your request is a common desire, and a search of Google will provide many ideas and horror stories. The bottom line is that only you know your applications and needs well enough to decide the appropriate tradeoffs, and there is no GOOD solution.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...