I want to combine the following two statements into a single sentence...
I have kept this thing between us.
I will keep this thing between us.
Which version is correct?
1: I have kept and will keep this thing between us.
or
2: I have and will keep this thing between us.
I'm not sure if OP's second "single sentence" version is technically a zeugma/syllepsis, but it's in that general area (the "deleted" main verb after have is kept, which doesn't match the "retained" form keep).
In practice native speakers do sometimes use such forms, but as a rule they're avoided. OP should use the first version, explicitly retaining the two different (past and present/future) verb forms.
Probably for purely stylistic reasons even OP's first version sounds just a little strange. Most native speakers would probably prefer to separate the two tenses even more explicitly...
I have kept this thing between us and will continue to do so
To illustrate exactly why the construction is problematic, consider...
"I have and will give you money"
...which could be interpreted as "I have given you money [in the past] and will do so again [in the future]" or "I have money, which I will give to you" (perhaps I never had money before, or never gave any to you).
I don't think the phrase "I have and will" works well in conjunction with a verb. The contexts where I would consider it useful are those where the verbs are relate to verbs specified elsewhere. In response to a question of "Have you done, and will you keep doing, some particular thing", it might be reasonable to respond "I have and will"; even though "have" and "will" are both bound to forms of "to keep", they tie back to the different forms of the word in the question.
Note that the same "I have and will" answer could have followed "Have you seen the ad for Acme Vacations, and will you be booking with them--a question with two different verbs that have a possible causal relationship; saying "I have and will" would imply that I am booking with Acme Vacations because I saw the ad. If the response had been "I have, but I will anyway", that would indicate that I had seen the ad, and would be booking the vacation, but the the ad did not motivate the booking; indeed, it made me less inclined to book with Acme.
'I have kept thing thing between us' does not mean that I got it!. It means that till the sentence is spoken, the listener has kept that thing between them.
So, the sentence I have kept and will keep this thing between us will convey the message that the speaker have been keeping the matter to him till now and then, in future, he'll continue to do so.
But if you are looking for something that confirms that the listener has understood the seriousness of the matter, the listener would probably say... "Yes, I got it." And considering this, I'll give my opinion here.
Not sure but I have heard this idiom that not only talks about the assurance of the listener but also the commitment. It is the idiom carry a secret to the grave.
"Well, I'll carry this truth/secret/talk/matter to the grave."
This says three things...
