Load Balance With 1 PPP gw and 1 Route

Hi, i would like a little help here with Load Balance..
I have routerboard 450G
First i would like to know if what i want to do is possible..

I have connected a DSL modem (bridged) at Ether1 and the connection is with PPPoe.
Everything works fine here…
I have another modem connected at Ether 2 (But cannot work with PPP ..only with a static route because it’s my neighbours modem..)

LAN Network at Ether3

Can i have a Load Balance with Failover
Everything routes through Ether1-PPP and when fails switch routing through Ether2 …?
Also can i have a rule to enable it only when i want to use bonding (to get double speed from both lines)

I found different guides on how to do load balance but didn’t make it to work because my Internet line uses a dynamic IP so most of the guides where saying to enter on some rules a static ip at the outgoing rules..