Splashtop Remote Desktop (non-HD) for Android version (including for NOOK Color) has supported the dual monitors feature since version v1.6.0.x.
NOOK Color is based on v126.96.36.199 and the dual monitors feature has been integrated into this version. However, NOOK Color does not yet support the three-finger touch. So, currently, there is no way to bring up the control UI to enable the dual monitors feature. Therefore, the dual monitors feature cannot be used on NOOK Color.
This is a technical limitation due to NOOK Color not supporting the three-finger tap, and we do hope that the new NOOK Color can and will address this issue.