Can I combine the power of 2 solar panels with 12V dc barrel jacks?
The splitter should give acceptable results if both panels have the same output voltage (ie: both are the same model and both get about the same amount of sunlight at the same time).

If they aren't evenly matched the weaker solar panel may leech energy from the system - such that just using the single better solar panel could be more efficient. series diodes on each would prevent that but reduce output by about 5% from just using the single panel.

To determine what to do you need to make some measurements: connect one panel and measure the voltage on both panels,  the swap panels and measure again, if in both cases the unconnected panel has higher voltage the the connected panel there will be some gain to to connecting both panels to the powerbank.

If not there won't,

In cases where the illumination is uneven, (eg: one panel faces east and the other west) then series diodes could help.

But since your panels already have diodes, there's nothing you need to do, just hook them up. there's no way that having two panels connected would be worse than having one.