Not possible. This is called bonding and your service provider has to support it. Some do but they supply there own routers. Closes is with PPC but the max speed will be 4 Meg although with some download managers it might appear as though you run 8 Meg because they are using both lines.
VERY Possibly if you have a concentration point outside to combine the lines again (and then use an IP address from there). Sadly Mikrotik decides to ignore the base technology to make that simple (MultiLink PPTP, including a server)
Withtou that you need to:
* Estbablish multiple PPTP links that appear on different adsl lines (easiest: hardcode /32 routes, use different target ip addresses on concentrator)
* Either use nth rules on both ends to distribute traffic over the links, OR put an EOIP / VPLS links over each, then use ethernet bonding.
PFsense and some others supposedly handle that a lot better, but Mikrotik does ignore this area. My biggest complaint - I would love to see proper PPTP handling on both sides with 5.x / 6.x. Like you I am in a similar situation - for an office where the biggest connection I can get is pathetic, sadly