"In this scenario is there a different way other than PPP to bundle the links?"
As an actual bundle of two serial links, I don't recall there being much if any other choice(s) beyond, as you have routers, using two routed links and doing packet by packet load sharing (the latter, packet by packet, is usually highly recommended NOT to use).
"If so what's the typical reason of using PPP over the two?"
PPS will load balance packets across multiple links, providing, often, a very equal physical load average on each link. It will further insure, far side doesn't re-order a packet flow's sequence of packets (unlike routing packet by packet [why it's NOT recommended]).
You might also do "ordinary" multiple link routing, but then, your link usage, as least short term, isn't well balanced. For example, a single flow will only use just one link (PPP will balance across all links). Multiple flows, may, or may not, use multiple links.
The big disadvantage of PPP, is the additional overhead it introduces, both for link bandwidth, and/or CPU consumption on both routers on both sides of the PPP links. (Typically, as classical serial links are "slow", i.e. low bandwidth, getting the most effective bandwidth from them was a great trade for the resource consumption of PPP. As serial links have been, now a days, often, replaced by much faster, i.e. higher bandwidth, links, "ordinary" routing is often used now, or "ordinary" routing is done directed by something like PfR which moves multiple flows about to load balance your links. [PfR, though, I recall, will still not redirect a single flow across multiple links as would PPP.})
