First it is important to know which controller you have. The SAS 6/iR can only do RAID 0 and 1 (and JBOD). The PERC 6/i can do what you are wanting. RAID can be configured across the connectors with no problems.
Hello, I have a PowerEdge 2900 III with SAS 6/iR Integrated (Embedded) and SATAu with two bays of 4 HDD int each bay. I would like to install 8 SATA HDD of 1TB each and to make a RAID 10 configuration for all 8 HDD so in total I will have one storage of 4 TB. Is it possible to do so as the PERC 6/iR has two separate connectors? Thanks, AG   
PowerEdge 2900 III RAID 10?