Having recently learned some basic structural calcs, I'll give you the hows and whys before the whats.
A beam (lintel, joist, scaffold plank, floor board... etc) has a property called 'Second Moment Of Inertia' which is that beams shape and sizes ability to resist bending. The unit for SMOI is given as mm^4 and for a square / rectangular beam is given as:
(W x (H^3)) / 12.
As you can see from that, if you double the width (such as bolting 2 joists together), you double the SMOI. If you double the height, you increase the SMOI by 8.
Once you have a beams SMOI, you can calculate the maximum center deflection by using the calcs on this website:
http://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html
The 'Youngs Modulus' or 'Modulus of elasticity' for steel and C16 (structural) timber are around:
Timber: 6,000N/mm^2
Steel: 200,000N/mm^2
As you can see, and as makes sense, Steel is much stronger than timber.
If you sandwedge a 6mm piece of steel between 2x 220x47. You get the equivalent deflection figures as a:
47+47+((200000/6000) x 6) = 47+47+200 = 220x294
So with minimum increase in overall size of the joist, you make it over 3 times stronger.
Alternatively, if you run the maths for a piece of I beam steel or box section steel (that you could fit between the joists) using these calcs for SMOI:
http://civilengineer.webinfolist.com/str/micalc.htm
you may find you can get away with installing steels between the existing joists.
So, really, your options would be:
1) Sister the joists up with steel between them (there is a fancy word for them that escapes me.
2) Install a steel beam across the room to split it into 2x 3m spans.
3) Install additional steel beams (probably I beams) between the existing joists for the whole span
Of all of these, 3 would probably be the easiest, but even then, 6m is a damn big span and this is going to be a BIG job regardless of how you go about it.
Unless you have the money to spend, I would consider keeping a pillar next to where the door is and the horizontal beam.
As to what the wife says. I tell mine "you are in charge of how it looks, but I am in charge of how it's build". It's better to have a slightly ugly room than a pretty pile of rubble.
!!!PLEASE NOTE!!!!
I am not a structural engineer.
I'm supplying this information for informative purposes only so you understand the situation.
There WILL be other factors to consider.
Please get a structural engineer in because this is a prime opportunity for massive structural failure. This may seem hypocritical considering my current project, but worst case if mine fails, the roof will bow and leak. Worst case for you, the house could come down.
Fubar.