Be careful about terminology, Bonding implies you have a single transparent pipe carried over two or more connections. This can only be done in two ways, the best way is using ML-PPP with an ISP that supports it, the problem is MT doesn't support ML-PPP! The only other way to Bond is to use EoIP tunnels, but that needs EoIP to be supported by the ISP, I've never heard of one that does.
If all you want to do is create a VPN across the Internet, then Bonding EoIP tunnels will work as you can install MT routers at both ends of the VPN and the ISP will route transparently.
The only realistic option you have for improving Internet bandwidth is Load Balancing where sessions are alternately allocated to the two connections, the problem is that no one session can exceed the speed of a single connection and the method of allocation of sessions is arbitrary so the connections are unlikely to each take exactly 50% of the traffic.
A request has been made to MikroTik to support ML-PPP, if you wish to help get this feature added, visit the new features Wiki page and vote for it.
In the meantime the only way to do true transparent ML-PPP bonding is either to use a generic Linux platform or use a specially engineered version, e.g. BondedCD http://www.upstreaminter.net/fm/
. Some people can achieve up to 20Mbps download and 2.4Mpbs upload over 4 x 8Mbps ADSL lines, but actual results appear to have some limitations in some locations in the UK. I have a BondedCD installation with 3 x 8Mbps connections, this only gives 5.5Mbps download, but I get 1.6Mbps upload, this link is primarily for VPN traffic, so although higher download bandwidth would be nice the Bonding is providing good upload speeds.