In a small saucepan, combine the cherries, raspberries, and 1 tbsp of sugar. Cook over medium heat for 8-10 minutes, stirring occasionally, until the berries have broken down and the sauce has thickened. Remove from heat and let it cool completely.

Line a 9x5 inch loaf pan with two layers of plastic wrap, leaving a few inches of overhang on the sides. This will help you lift the semifreddo out later.

Create a double boiler by filling a saucepan with an inch of water and bringing it to a simmer. In a large heatproof bowl, whisk the egg yolks with 1/4 cup of sugar. Place the bowl over the simmering water and continue to whisk constantly for 5-7 minutes, until the mixture is pale, thick, and has doubled in volume. Remove from heat and set aside.

In a large, separate bowl, use an electric mixer to whip the cold heavy cream with the vanilla extract until stiff peaks form. Be careful not to overwhip.

In another clean, large bowl, whip the room temperature egg whites on medium speed until they become foamy. Gradually add the remaining 1/4 cup of sugar and increase the speed to high. Whip until stiff, glossy peaks form.

Gently fold the egg yolk custard into the glossy meringue until just combined. Then, gently fold the meringue mixture into the whipped cream in two additions, being careful not to deflate the mixture.

Pour half of the semifreddo base into the prepared loaf pan. Drop spoonfuls of half of the cooled berry compote over the top. Use a butter knife to gently swirl the compote into the base. Pour the remaining semifreddo base on top, followed by the rest of the compote, and swirl again.

Cover the loaf pan with plastic wrap and freeze for at least 6 hours, or overnight, until firm.

When ready to serve, use the plastic wrap overhang to lift the semifreddo from the pan. Let it sit at room temperature for 5-10 minutes to soften slightly before slicing and serving.