If your antenna system is good for ADS-B frequency range, and it's already installed, there should be no obvious reason why you shouldn't use it. There are some considerations though.
The splitter doesn't matter much, as it's wideband, and should compensate for impedance and attenuation concerns by design. But, are the antennas designed to cover 978 and 1090 Mhz? If not, you'll need a different antenna(s). If the antenna is too broad ranged, it can cause problems in a multi-antenna installation by introducing interference. Technically the interference will be there in a single antenna system too, but dual antennas allows it to affect your reception in additional ways. However, you are probably okay in that particular regard if the system worked well before.
Having identical length cables is a measure used to reduce phase or spatial interference in multi-antenna systems. If the same signal hits both antennas, but then arrives at the splitter or receiver at slightly different times, they can effectively cancel themselves out to some degree. Having identical length cables minimizes this, but can't eliminate it completely. That's because, due to angles and atmospheric issues, the same signal often doesn't hit both antennas at exactly the same time.
Impedance can bite you in the butt too. If the impedance of the antenna too closely matches or exceeds the impedance of your cables, combiner and receiver, you could have problems. This gets highly technical (scientific), so I'll just say it's probably much easier to hook it up and see if it works than it would be to calculate all the scientific formulas. It most probably will be fine.
As I said, although there are things to consider, none of this will likely prevent you from doing what you want to do. Most other folks I've seen using a two antenna system on a small aircraft say using two antennas doesn't make that much of a difference. Apparently it's more effective on larger aircraft. But it has some benefit, and you have the advantage of a system that has already worked in some capacity before, so some of these things have presumably already been addressed in the design. And since it's already installed, you have the best advantage of all. You can simply hook it up and see if it works without having to brainstorm a design in advance. It's a receive only system, so there's no harm in trying it out. I'd go for it as long as the antennas are good for 978 and 1090 Mhz.
