There are two types of FRR- NHOP and NNHOP. Next-Hop (NHOP) is used to protect a link and Next Next-Hop (NNHOP) is used to protect the next-hop.
In your case, the PLR (Point of Local Repair) is R1-router. However, the MP (Merge Point) is unknown. Here, both (Primary and Backup) tunnels are two different tunnels and the second tunnel can be made to work as a Backup tunnel but only in case of NHOP.
If your destination is connected to R4-router, I think you should implement NNHOP- protecting R2-router with R3-router as the Merge Point. The Backup tunnel will be through R1-R5-R6-R7-R3.
You would have to configure 2 TE Tunnels with Explicit paths as R1-R2-R3-R4 as primary and R1-R5-R6-R7-R8 as backup. To get superior fast convergence, I suggest you configure IS-IS as the IGP, as this is tweakable to easily acheive sub-50ms and even faster. I have implemented such solution and by tweaking the spf interval to the min (spf-interval 1 1 1) and also configured fast-flood to 2, this ensured extremely fast convergence and FRR failover and Fallback (even more important than failover). Of course configuring these tweaks, you'd have to ensure you have a very stable network, as such the implementation I ran was between 2 Data Centres with DWDM 10G links between them.
Yes that is acheivable, if you look at the example configurations I gave you, and have your IGP tweaked (IS-IS for best resuls there), then having a backup tunnel with sub-50ms failover is very acheivable.
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