Since they’re on completely different channels, there is no concern with mixing LED’s for the LT1. In contrast, LED’s with different forward voltages on the same channel in parallel would cause the lower voltage LED to run brighter.
So it really depends what you want on each channel.
If you stick with the 5000K LH351D, you have a good balance between brightness and CRI, but probably a slightly green tint (per Maukka’s measurements of the LT1). If you swap to 5000K 519A, you take a very slight (imperceptible?) brightness reduction, get a slight increase in CRI, and get neutral tint.
If you slice the 5000K LH351D, you’ll probably a little under 4500K with close to neutral tint (per thefreeman’s measurements of various slicing heights) and a slight decrease in brightness. A sliced 5000K LH351D or a dome-on 4500K 519A will likely be very similar all-around. I’d personally favor the 519A in this case - slightly more neutral, slightly better CRI, but the the LH351D doesn’t become worse simply because you have 519A on the other channel.
If you want rosy tint at all color temperature mixes, a dedomed 519A would be the way to go, but because the color temperature drops so much for this LED when dedomed, even starting with a 5700K would likely end up below 4500K. I think if I did this, the result would be that mixes of the warm and cool channels would end up too rosy for my taste.
The slicing shouldn’t change the light distribution from the LT1 noticeably. If you look at the datasheet for an LED that has both domed and dedomed options from the factory like the XP-L (datasheet page 16), you will see a slight difference in the relative spatial distribution at moderate angles, but the more important difference in the distribution you get from an optic is the difference in the apparent visual size when there is a dome changing the apparent source of the light rays.