In the DataManager, isolate the [YearMonth] field (in a new Field Organiser block, delete all other fields), de-duplicate, and add [RankMonth]=Rank([YearMonth]) field, merge then on the same field with the rest of your dataset.
Result will be evaluation of the Month values as unique rather than multiple values, so all records that have Jan, Feb, Mar in the same year will have values 1,2,3, even if you have multiple records in each month.

Add 2 new formula fields, that refer to this rank

CM=
DECLARE(
CurrentMonth, SUBSET_MAX([RankMonth]),
SUBSET_SUM([Sales], SUBSET2([Date],[RankMonth],[Date],CurrentMonth)))



LM=
DECLARE(
CurrentMonth, SUBSET_MAX([RankMonth]),
SUBSET_SUM([Sales], SUBSET2([Date],[RankMonth],[Date],CurrentMonth-1)))

Split the Bar/Line view on the [Day] and use [CM] and [LM] as measures, side by side.

See attached demo.

You can also try this solution (please see file attached)

Great! Thank you. I will check both solutions.

That is a good idea Bart, thanks!
The only issue is that you are relying on TODAY function, so at the beginning of each month the data might not be there e.g. we're in January now, but your latest sales data might be from December.

IF(
[YearMonth]=DATETOTEXT(TODAY,"yyyyMM"),"Current Month",
[YearMonth]=DATETOTEXT(DATEADD(TODAY,-1,"month"),"yyyyMM"),"Previous Month",
null)

SubsetMax will pick up the latest month in the dataset, so might be more reliable.

Ok so what if I want the user to select a month and the view shows the selected month and previous month? Is there a max-function that applies to selections?

Edit: A max-function like sum(Amount where max(RankMonth))

If you select month n and want to compare it to the previous one (n-1), then n month is Max value for this subset.
You could use a variable to select the month ( [YearMonth] field to prevent 'overspill' and errors between years)

Example 2 with header attached. Could not delete "example" file in edit mode.
Note: formula used for [Current/Previous]=

IF(
[YearMonth]=DATETOTEXT([Month of report],"yyyyMM"),"Current Month",
[YearMonth]=DATETOTEXT(DATEADD([Month of report],-1,"month"),"yyyyMM"),"Previous Month",
null)

[Month of report] is the variable, so month n is selected by the end user, while the values on the chart and all other views are re-calculated on the fly.