It is very common for agile teams, especially Scrum teams, to estimate both their product backlog and sprint backlogs. In this article, Mike Cohn will address:
- Why estimating both the product backlog and sprint backlog can be useful even though it seems redundant
- Why teams should estimate the two backlogs in different units
- When teams should estimate
- Whether all teams should estimate
Although I think you should not try to use hours for estimation, I agree using different valuations for an estimation is best. I find using T-shirt sizes for PBIs and Story Points for SBIs work well.
Read the complete article here, and also take note of some excellent comments at the bottom: https://www.mountaingoatsoftware.com/blog/why-agile-teams-should-estimate-at-two-different-levels