Multiple Internet Connections.

Hey All,

I currently have a network comprised of 4 internet connections, and each interface serving a different role.
A-(1)Site-Site vpn (Site A)
B-(1)Web traffic
C-(1)Site-site VPN (Site B)
D-(1)Not used ATM.

Each Line is only a 1.5mb UL/DL, and I was wondering if it's possible to combine a+d to make a 3MB line. Or to use D when the saturation is high on A,B or C.

Thanks,
120Newb