Yes it is very possible. You need to upload both excel models using Get Data. Then you can create reports/charts against each data set. Then you go to Dashboards and create a new one. Make sure you are in the new dashboard by clicking on its name, then you can click on the reports in the report section, mouse over charts you need in your dashboard and click on the Pin icon. Now you will have those charts pinned to your newly created dashboard and when you click on the dashboard tiles, you will be taken to the corresponding reports.
You cannot have two different datasets drive the same report however.
You can add calculated fields to datasets in their source data model, not after upload. So in Excel, if you used the Power Pivot data model, you can add calculated fields and calculated columns.
Reports are designed to be pulled from a single dataset while dashboards are the mechanism for combining visualizations from multiple datasets.
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You can bring in tables from any source into power query and then combine into one model (which ends up being a dataset in Power BI). A great way to do this is to use M and the advanced editor in Power Query to mash up sources. You can do it manually in Power Query and then look at the advanced editor in Power Query (either in Excel or Power BI Desktop) to understand how it generates the M for each table or mash up.
check out this video to get an idea of what's possible