How to "multicast" an async iterable?

This is not easily possible. You will need to explicitly tee it. This is similar to the situation for synchronous iterators, just a bit more complicated:

const AsyncIteratorProto = Object.getPrototypeOf(Object.getPrototypeOf(async function*(){}.prototype));
function teeAsync(iterable) {
    const iterator = iterable[Symbol.asyncIterator]();
    const buffers = [[], []];
    function makeIterator(buffer, i) {
        return Object.assign(Object.create(AsyncIteratorProto), {
            next() {
                if (!buffer) return Promise.resolve({done: true, value: undefined});
                if (buffer.length) return buffer.shift();
                const res = iterator.next();
                if (buffers[i^1]) buffers[i^1].push(res);
                return res;
            },
            async return() {
                if (buffer) {
                    buffer = buffers[i] = null;
                    if (!buffers[i^1]) await iterator.return();
                }
                return {done: true, value: undefined};
            },
        });
    }
    return buffers.map(makeIterator);
}

You should ensure that both iterators are consumed at about the same rate so that the buffer doesn't grow too large.