Electrical installation regs in the uk state that any electrical work "...should only be carried out by a suitably trained and competent person...."
If you're at all not sure, then you should find an electrician.
If you're using the correct cable, and connectors/fittings, then there should be plenty of room to get 2 wires in one connector. Just make sure the wires are not too short or too long. Don't bend the cable at sharp angles, where possible, bend it in an arc or a u shape.
As for your arms aching, it's probably because you're moving/stretching in ways that your body's not used to.