If you have paid for two units, and want to combine the payments into one then its totally at the discretion of the developer to offer you this option.
If the building has been delayed indefinitely and no construction is going on then you can discuss with your developer regarding this option and they should cater to your request.
If the construction is going on as normal then its very unlikely that the developer will agree with you.
You can always cancel one apartment, as per the law the developer would need to refund 70% of your payment. So it would be in developer's interest to combine the 2 units into one.
I hope this helps, have a discussion with your developer and let us know. Would be able to guide you better after you have had a talk with the developer.