Neither the T3400 nor the T3500 support SLI.

The Dell Precision T3400 uses
Chipset Intel X38 Express
Processor Socket LGA775 Socket
This model has 2 video slots, however, it does not support SLI.
Crossfire Only.

https://www.youtube.com/watch?v=3aE-U3Bsz7s

Many of the older Dell Models of this same spec like the XPS 400/Dimension 9150 will support Pascal Based Nvidia cards or Radeon HD series up to HD8000 series.  However, the R series cards like the RX460 for example do not work due to bios needing DOS VESA MODE 103.

https://www.youtube.com/watch?v=qa9b0TkTsLM