Search data in multiple tables

Solution 1:

If I create separate indexes for tables, can I search the data in both indexes from single search API? Is it possible to join 2 indexes? Just cross checking if such functionality exists.

No. Search is only limited to a single index. You would need to combine the search results from multiple indexes on the client side.

Another option I am aware of is – load data from both the tables to single index using solution given in below. Is this possible only via .NET API? Can we develop it from Portal?

You should be able to do it from the portal. Essentially the idea is to create two data sources (one for each table), two indexers (one for each data source) and have these indexers populate the data into a single index.

Another idea would be to create a database view that combines the data from these two tables and use that view as the data source for your index. That way you don't have to create separate data sources and indexers.