Multiple iif statements

I am trying to combine two iif statements and I am trying enter the correct iif statement for it. This is what I have so far.

Original iif statement that exists
IIf([ORIGINAL_LOCK] Is Null,[new_App_DT],[ORIGINAL_LOCK])

I am trying to add another iif statement to the existing iif statement with the following methodology.
Iif original lock does not fall within the filter below, then change [ORIGINAL_LOCK] to [new_App_DT] otherwise leave as [ORIGINAL_LOCK]

Between [Forms]![DateFilter].[dtStart] And [Forms]![DateFilter].[dtEnd]


Would the new combined iif statement look something like this?
(IIf([ORIGINAL_LOCK] Is Null,[new_App_DT],[ORIGINAL_LOCK]),
IIf([ORIGINAL_LOCK] is not Between [Forms]![DateFilter].[dtStart] And [Forms]![DateFilter].[dtEnd], [new_App_DT], [ORIGINAL_LOCK]))

Thanks for the help