Since you haven't made a purchase yet, may I suggest the T620 instead. It will support dual H710P controllers:
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Dual 16 x 2.5” | 16 + 16 | Dual PERC H710P | SATA HDD/SSD SAS HDD/SSD |
The T420 comes with either the backplane or cabled drives, but it can't be converted.
Regards,