If you must have separate viewing your simplest bet its to buy two DVD players. Your youngest should probably have a headrest screen, controlled from the front seat, but the elder one could cope just add easily with a laptop version.
Also, when considering sound, your 6 year old should manage headphones but at 3, some do and some don't. Either way, ensure you have control of maximum volume!
There are so many on the market it will probably cone down to a cost decision.
We actually found there were enough films on the market with wide appeal that we could provide our 3 (with 7 years separation) enough videos for entire holidays (eg Toy Story, Cars etc.) so we only got one portable DVD player for the 3 of them.