I really like the throne sharing Jesus demonstrates here in Rev 3:21. His throne seems clearly to me to be a DIFFERENT throne, but that does not mean that he is bolted to it! This is also useful context in understanding the subsequent passages that talk of the lamb on God's throne.
I'd also just like to point out that I personally find this throne sharing idea quite inspiring. It goes against the grain of the way systematic theologians like to rationalise and compartmentalise the Bible in some rigid way (or rather our own theological worlds). This throne sharing to me speaks of trust, intimacy, entrusting, even discipleship. It is dynamic. Maybe it is also the apocalyptic genre but it seems vibrant and alive!