Hello!
I like the stroller you have chosen for your twins. The Bugaboo Donkey is indeed a practical investment in the long-term and even will come in handy later, when you decide to sell it. With the black color currently in decline, the stroller is available mainly in melange with a dark blue as the main color. When it comes to the Twin set, you can either add to the Mono version (I will explain later) or ask a retailer around how to do it. The second option of going to the store where the Bugaboo brand is sold is probably more comfortable because, with such an investment, people in the store will be all over you. You won't be short of advice on colors, accessories, or anything else you may need and leave them to it. I recommend contacting the authorized Bugaboo dealers in your area. You can give them a ring or email them, but ideally, you want to see the stroller in person. They can give you a demonstration, help you to create the version you are after, and assist with an order.
If you are still keen to create the Twin version yourself or want to understand what else you need to add to the Mono version, I will start from the beginning. Yes, the Donkey (in the single version) is a 2in1 stroller system consisting of a frame that accommodates the fabric of the carrycot as well the seat unit, plus an additional shopping basket usable in the single version with one seat or carrycot (and, of course, the conventional basket beneath the chassis and hood). You will need another seat unit and carrycot, i.e., you are buying a seat - 'Set for the second child' in which you have a seat frame and the seat itself with a hood. PLUS, you also need a 'Carrycot Set', which comes with fabric for the carrycot plus a mattress and an apron. Put all of these components together, and you will have a complete 2in1 stroller system for two children - twins, plus an extra shopping basket, handy if you decide to go somewhere with only one child.
Subsequently, you also need to buy a double adapter for car seats - I recommend the Maxi-Cosi/Cybex type. Thus the Britax Romer is also a possibility, and last but not least, two car seats of your choice. According to my experience, the Maxi-Cosi Cabriofix or Pebble will be used for the longest time, but choose according to your preference and budget (the Maxi-Cosi adapter is compatible even with widely available, cheaper, but still excellent Joie Group 0+ car seats).
The optional accessories I would recommend are the rain covers, especially if you are planning to use the strollers even in bad weather. The Bugaboo rain covers are more expensive but quite outstanding. Another excellent choice is the footmuffs. The Bugaboo footmuffs are superb. They are warm, and the fit is exceptional. However, if you would like to save some money here, the Lodger footmuffs are excellent too and fit BGB strollers perfectly (both BGB and Lodger strollers have an L-shaped seat, so Lodger footmuffs offer a better, no-slip grip, plus the quality is really great as well). Check the website for current sale offers. They offer delivery to Slovakia too, so you can get the Lodger Bunker much cheaper, whether made of fleece or the shiny ones with patterns.
Finally, one more option to mention – you can place an order directly on the Bugaboo website. If you check the Donkey Twin section, black is available in some versions. (if you need to change the country, click on the top right corner at the bottom of the page). Simply click on what you need, and you will get your dream Twin stroller. You don't even need to pay for the delivery service with such an expensive thing, but I am not sure about tax/duty payment in your country - take it into consideration to not be surprised. There is a chance you will have to pay a certain sum. However, I believe people in your local store will be willing to help, and it won't be a problem for them to order a black Donkey Twin for you if it is currently available at BGB - even if not on their website.