Excerpt from this link http://blog.3g4g.co.uk/2011/02/volte-semi-persistent-scheduling-sps.html
says, "dynamic scheduling of best effort data can occur on top of SPS, but the SPS allocations would take precedence over any scheduling conflicts"
The UE finds its PDCCH by monitoring a set of PDCCH candidates in every subframe. This is known as blind decoding. The UE demasks each control candidate's CRC using its RNTI. If no CRC error is detected, the UE considers it as a successful decoding attempt and reads the control information within the successful candidate.
With the possibilities of different RNTIs, PDCCH candidates, DCI and PDCCH formats, a significant number of attempts may be required to successfully decode the PDCCH. To overcome this complexity the UE first tries to blindly decode the first CCE in the control channel candidate set of a subframe. If the blind decoding fails, the UE tries to blindly decode the first 2, 4 then 8 CCEs sequentially, where the starting location is fixed for common search case. (Courtesy http://www.mathworks.in/help/lte/examples/pdcch-blind-search-and-dci-decoding.html)
In summary, UE needs to search for :
 (Temporary) C-RNTI in UE specific search space
 SPS C-RNTI in UE specific search space if SPS is configured
 SI-RNTI/P-RNTI in Common Search space (not talking about RA-RNTI as it only happens during Random access procedure)
 TPC-PUSCH-RNTI in Common search space if configured
 TPC-PUCCH-RNTI in Common search space if configured
UE specific search space has 6(aggregation1) + 6(aggregation2) + 2(aggregation4) + 2(aggregation8) = 16 candidates.
Two possible RNTI in UE search space shall make searching (worst case)= 16*2=32
Common Search space has 4(aggregation4) + 2(aggregation8) = 6 candidates.
Three possible RNTI in common search space shall make searching (worst case)= 6*3=18
SPS and C-RNTI can occur at the same time, and with the fact that SPS-RNTI based decoding is only used for activation/reactivation of SPS bearers and it uses C-RNTI other times, 2 RNTI search spaces are not that bad.