How to Have Two Different Instruments Play the Same Rhythm in the Same Measure.

I don't think you can combine two different instruments on the same staff. (But I'm not sure!!!) If not, then you shouldn't implode the two staves. I think the sound happens at the level of the staff.

You wrote:
I am writing a percussion part and want the part to have the same rhythm played on bass drum and splash cymbal.

Okay...
This is a percussion part - which has "the same rhythm played on bass drum and splash cymbal":

It is comprised of a drum kit. The upper notes are splash cymbal, lower notes are bass drum.
.

But then you also mention parts:
When I implode the two parts, the splash cymbal part converts to bass drum...

Hmm... Now you're also writing parts? This shows two parts:

There is a staff for the splash cymbal part, and there is a separate staff for the bass drum part. Therefore, together these comprise 2 parts, as shown.
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So, are you trying to combine two separate (and different) parts/staves into a single staff?
Please attach your score and explain precisely what you are trying to accomplish.