I know two ways:
1-) First Form:
You could get a machine on AWS, Azure or Digital Ocean and install the two applications on it and mongodb as well.
In this video the guy explains how to deploy a nodejs application with free SSL and explains some parts of the server's security as well:
link
To make the / admin issue redirect to another app, just see the nginx rules to redirect to the other application.
Using this video I deployed a Nodejs + Mongo app on AWS and it worked fine.
2-) Second Form:
You can use share hosting, which is the much-used shared hosting for small sites in PHP, has an answer in English that explains how to do:
link
The author of the best response created a project called node.php in github, it might be interesting to take a look at the project and maybe even the mongodb you can install that way.
I've never used nodejs in this way (share hosting), I used a machine in the AWS free tier and it answered me for a specific project, in digital ocean you spend $ 5 and have one or two months free and in AWS you have a year free.
It's up to you to assess the need for the application, such as the number of users, requisitions in the database and other things, because AWS is expensive, free tier machine after one year are charged $ 20 a month, digital ocean are $ 5, but I think it's a more modest machine and you have to consider which are the two app + a bank, maybe it's interesting to get a machine just for the database.
Edit:
Umbler has a nice price to start (6 REAIS per application in the most basic package of them PP) also and they have hosting nodejs + mongodb, it pays to take a look at tb.