How about using a subquery with the two dates?
Here is an example using an imaginary sales cubes that selects the Amount value for a given Region (i.e., Europe) and two days only:
select
[Measures].[Amount] on 0,
[Customers].[Geography].[Region].[Europe] on 1
from (
select {
[Time].[Calendar].[Day].[8 Feb 2005],
[Time].[Calendar].[Day].[10 Feb 2005]
} on 0
from [Sales]
)
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