Dual graphics card T3500

I have a T3500, fairly stock except for a nvidia GT730 (dual DVI) running dual monitors.  It is sufficient for my 15-yr old and has been her PC for quite some time.  She is into drawing so we purchased a graphics tablet that uses an HDMI port and acts as a monitor except its run with a special pen specifically for drawing on the graphics tablet.  I replaced her GT730 with a GT750Ti (1 HDMI, 1 DVI).  Now we have this nice dual monitor setup but can only use 1 of the monitors.  Is there a way to use both graphics adapters so she can have her dual monitors in the 730 and just utilize the HDMI on the 750 and still have an open DVI?  I tried to install both but only one of them would actually work.  I couldn’t find anything in BIOS.  Read something that some motherboards have a switch to enable and some power stuff but neither card has external power port.  Didn’t know if I missing something or if it’s not possible and I’m banging my head in the wall.  Can I mix and match video adapters?  I do have some nvidia Quattro cards laying around too in various models I could try also.