OAuth: Access token per infrastrucure or access token per client?

We have two elements (clients) in our infrastructure:

1. First Client: A service that receives webhooks. Each time a webhook is received it tries to get information from dropbox.
2. Second Client: A user interface(.net) that allows to download files from dropbox.

Both services are accessing to dropbox.

Should we ask for an access token for each client or we can use only one for both.

It's important for us that you figure out that we want both clients are like a single infrastructure. We are trying to avoid that a user is requested for "the same" infrastructure twice.

We're considering that both clients as a single "client" even thought both are running in separated behaviors. So, what about if both clients are using the same access token?

I hope I've explained so well.