Is there a way we can merge the columns in DOMO table card?

Not on the normal table card, you can't merge cells. The card is pretty limited on any styling. You can find more capabilities on html tables and DDX bricks.

You can kind of simulate a merging if you use a Beast Mode calculation and concatenate, the big problem with it is that you loose all the cool features about number formatting, so you'll have to manually set those up. Here's an example of something I did not long ago (this inside a pivot table):

There I have a single column in the table that is displaying 3 metrics as a whole, a count, % of total and $ total, of course the beast mode to do these is not exactly the prettiest thing in the world 😅, just in case you might want to explore an alternative solution.

CONCAT('🏢:',COUNT(DISTINCT CASE WHEN Opportunity Link IS NOT NULL AND IsClosed = 'true' THEN account_id END),' (',ROUND(100*COUNT(DISTINCT CASE WHEN Opportunity Link IS NOT NULL AND IsClosed = 'true' THEN account_id END)/COUNT(DISTINCT account_id),0),'%) 💵: $',ROUND(SUM(CASE WHEN Opportunity Link IS NOT NULL AND IsClosed = 'true' THEN Incremental_Average_ARR__c ELSE 0 END)/(CASE WHEN SUM(CASE WHEN Opportunity Link IS NOT NULL AND IsClosed = 'true' THEN Incremental_Average_ARR__c ELSE 0 END) > 1000000 THEN 1000000 ELSE 1000 END),2),(CASE WHEN SUM(CASE WHEN Opportunity Link IS NOT NULL AND IsClosed = 'true' THEN Incremental_Average_ARR__c ELSE 0 END) > 1000000 THEN 'M' ELSE 'K' END))

This doesn't really merge the column headers while keeping the columns separated, which is what @ArborRose solution does great.