i want to know if there is any way of browsing the net and downloading using two connections (one is mobile internet with my phone as the modem and other is a landline pppope or something like that with an external modem.)
Useless to do so in the first place with the phone. You're speeds will be very slow over the phone. Waste of time.
Possible, yes with the right know how and equipment but only applicable really at the corporate level. Would need to get the isp involved.