Does EC2 network bandwidth apply per direction or as an aggregate limit?

The bandwidth specification applies independently to both inbound and outbound traffic.

According to the AWS official documentation:

"Instance bandwidth specifications apply to both inbound and outbound traffic for the instance. For example, if an instance specifies up to 10 Gbps of bandwidth, that means it has up to 10 Gbps of bandwidth for inbound traffic and, simultaneously, up to 10 Gbps for outbound traffic."

So for a 10 Gbps instance, you can use up to 10 Gbps inbound and up to 10 Gbps outbound at the same time — they don't share the same pool.

A few additional things worth knowing:

For smaller instance sizes, the listed bandwidth is often a burstable "Up to X Gbps" value, not a guaranteed baseline Single-flow throughput is capped at 5 Gbps between instances outside a placement group, regardless of the instance's total bandwidth ENA Express can raise the single-flow limit up to 25 Gbps within the same AZ

EC2 instance bandwidth specifications apply to both inbound and outbound traffic separately and simultaneously. This means that if an instance specifies up to 10 Gbps of bandwidth, it has up to 10 Gbps available for inbound traffic and, at the same time, up to 10 Gbps available for outbound traffic. The bandwidth is not a combined aggregate limit that must be shared between the two directions.

So in your example, a 10 Gbps instance can handle 10 Gbps of incoming traffic and 10 Gbps of outgoing traffic at the same time, giving you the full specified bandwidth in each direction concurrently.
Sources
Amazon EC2 instance network bandwidth - Amazon Elastic Compute Cloud

To add to what others already stated, the bandwidth applies to the EC2 and not ENI. That is, if you have multiple ENIs attached to an EC2, the overall bandwidth is still going to be 10 Gbps.