The same interfaces can be used for SvxLink as for the original Echolink program for Windows. But if you want to make your own, you can also separate the two channels and build an adapter to connect 2 radios to one soundcard.
The following is a small circuit diagram by Martin (DF1AMB):

With a stereo sound card and an RS232 port, SvxLink can control two radios independently of each other. Below is a simple interface that also allows you to send an external squelch criterion to SvxLink (if your RX provides the appropriate information).

Does an interface with galvanic isolation have to be used in any case?
No, if, for example, you connect the radio to the same power supply as the PC, the galvanic isolation has no effect anyway. For a long time we operated a link without isolation, where the radio was connected to the 12V of the PC and had no problems.

Is a hardware DTMF decoder necessary for SvxLink?
No, SvxLink has the following software decoders: DTMF, ring tone, CTCSS, SEL5 (ZVEI, EEA,EIA,CCIR,…).